1. Field of the Invention
The present invention generally relates to a compound semiconductor device and particularly relates to a process of manufacturing an optical semiconductor device used for optical communications and optical information processing.
A compound semiconductor has a band structure of a direct transition type that interacts with light and thus an optical semiconductor device utilizing compound semiconductor is widely used in the fields of optical communications and optical information processing. An InP material system's semiconductor device, particularly a laser diode, is important since it produces optical signals having a wavelength of 1.3 or 1.55 μm band which may be transmitted in an optical fiber.
2. Description of the Related Art
In order to improve laser oscillation efficiency for such a laser diode, it is necessary to provide a current blocking structure for confining injected carriers within a limited region along an axial direction. Further, since laser oscillation is produced by induced emission, light should also be efficiently confined within the region where the carriers are confined. For a laser diode of an InP material system, a horizontal light-confinement effect is achieved by adjusting a difference of refractive indices of the InGaAsP core for guiding the light and an InP buried layer.
FIGS. 1A to 1D are diagrams showing various steps of a manufacturing process of a laser diode 10 having a buried-hetero (BH) structure which serves as an electric current and light confinement structure.
Referring to FIG. 1A, a multi-quantum well layer 12 is formed over an n-type InP (n-InP) substrate 11. The multi-quantum well layer 12 includes repeatedly stacked InGaAsP layers. Further, a p-type InP (p-InP) cladding layer 13 and a p-type InGaAs (p-InGaAs) contact layer 14 are, in turn, formed on the multi-quantum well layer 12.
Then, in a step shown in FIG. 1B, a SiO2 film 15 serving as an etching protection layer is formed on the contact layer 14. Then, dry etching is performed on such a structure to form active layer mesa-stripes. In the illustrated example, the mesa-stripes extend in the <011> direction.
In a step shown in FIG. 1C, a metal organic vapor phase epitaxy (MOVPE) is performed using the SiO2 film 15 as a selective growth mask, such that crystals grow on both sides of the mesa strips to produce Fe-doped high-resistance InP buried layers 16A and 16B. During a regrowth step of such InP buried layers 16A and 16B, the (111) B surface develops which is a growth-stop surface. As a result, the buried layer builds up at the edge of the mask and gives a growth configuration that is raised as shown by reference numerals 16a and 16b. 
Finally, in a step shown in FIG. 1D, the SiO2 film 15 is removed, a p-side electrode 17 is formed on the contact layer 14 and an n-side electrode 18 is formed on a lower surface of the substrate 11.
As has been described above, when a buried growth process of the InP layers 16A and 16B is performed using the SiO2 film 15 as a selective growth mask, the InP layers 16A and 16B inevitably rises at the regions 16a and 16b which correspond to the edges of the SiO2 film 15. This is due to the fact that the crystals do not grow on the SiO2 film 15 and thus the concentration of the material locally increases on the SiO2 film 15. This causes an excessive supply of the material to the surface of the InP layer 16A or 16B grown on both sides of the mesa-region. For the step shown in FIG. 1C, when the height of the mesa-stripe is about 1.5 μm, the InP layers 16A, 16B will rise about 0.7 μm at the regions 16a, 16b at the edge of the mask.
As has been described above, in the step shown in FIG. 1D, the p-side electrode 17 is formed on such a stepped surface. When a Ti layer, a Pt layer and an Au layer are sputtered in turn to form the p-side electrode 17, the Ti layer and the Pt layer each has a thickness of only about 0.1 μm. Therefore, as shown in FIG. 2, a break or discontinuity of the electrode layer may occur at uneven parts 17a due to the stepped configuration of the underlying structure. Such a break of the electrode causes an uneven electric current flow and thus gives rise to electric degradation of the device.